The present invention relates to a compact and high power zoom lens system and, particularly, to a compact zoom lens system having small total length and suitable to use in a video camera or the like.
A number of compact zoom lens systems which may be used in a video camera or the like have been proposed. For example, as disclosed in Patent Application Laid-open Nos. 59-28121, 61-140913, 61-182011 and 63-34505, etc., a zoom lens can be constituted with a relatively small number of lenses and thus can be made compact with high performance by applying an aspherical surface to the zoom lens system and suitably regulating focal lengths of lenses constituting the zoom lens.
In an example of the conventional zoom lens for a 1/2 inch image size having a zoom ratio of about 6, however, a ratio of the total length of the zoom lens to a focal length thereof at the wide-angle end is on the order of 11 to 12. Such zoom lens is not small enough to use in a recent video camera which is much more miniaturized.
Generally, in order to reduce a total length of a zoom lens, it is necessary to lower zoom ratio and/or make F number larger. When, in order to reduce the total length without reducing its power, the whole zoom lens system is made smaller analogously, focal lengths of respective lenses become shorter and radii of curvature thereof become smaller, necessarily. Therefore, problems such as larger lens diameter, difficulty of compensation of various aberrations, increased number of lenses and severe manufacturing tolerance of lenses, etc., may occur.
As mentioned before, when the total length of the zoom lens is reduced, various aberrations are enhanced and the desired performance can not be obtained. Further, in order to compensate for these degraded aberrations, it is necessary to increase the number of lenses, resulting in a necessity of providing a space for the increased lens number which may require an increase in the total length. Therefore, it is very important how to compensate these aberrations resulting from the shortened total length a minimum number of lenses.
An object of the present invention is to provide an improved zoom lens system which includes a minimum number of constituting lenses with good compensation for aberrations and which has a high power with a zoom ratio being on the order of 6, an F number of 1.4 to 1.8 at the wide-angle end, a ratio of a total length of the zoom lens to a focal length at the wide-angle end being on the order of 7 to 9.